搜题请搜索小程序“爱下问搜题”
问题 选择题

如图所示．质量为m的球放在倾角为α的光滑斜面上，当挡板AO与斜面间的倾角β为多大时，AO所受压力最小（  ）

A．300

B．450

C．600

D．900

答案

答案：D

题目分析：对球受力分析如上图所示，由图知两支持力的合力与球的重力平衡，当F2与F1垂直时，支持力F2有最小值，即挡板AO与斜面间的倾角β为900时，AO所受压力最小，D对，ABC错。

点评：学生要画出球的受力示意图，再用三力平衡的知识、平行四边形定则和直角三角形的边角关系去分析问题。

单项选择题

门座起重机动载试验所用重物的重量为额定重量的（）%。

A、130

B、120

C、110
查看答案
填空题


The United States counts its population every ten years, and each census reveals that the racial and ethnic mix is changing dramatically, so by the year 2050, the "average" American will not be descended from (1) , but the majority of U. S. residents will trace (2) ancestry to Africa, Asia, the Hispanic world, the Pacific Islands, or the Middle East. Once the United States was a microcosm of European nationalities, (3) the United States is a microcosm of the world. The United States is (4) considered a "melting pot" society by many of its residents. (5) , many people prefer the term "salad bowl." They use this (6) to describe American society. American society will soon be (7) nonwhite. "Melting pot" implies that the different ethnic groups blend together (8) one homogeneous (9) , "salad bowl" (10) that nationalities, like the ingredients in a mixed green salad, retain their cultural (11) . Earlier generations of (12) believed they had to learn English quickly not only to survive but also for (13) .
Now, many immigrant groups do not (14) the same need. Because there are many places in America (15) you can work, shop, get medical care, marry, divorce and die without knowing English. (16) , Chinatown in San Francisco and New York. Also, Los Angeles has many Vietnamese immigrants and immigrants from Mexico. (17) , many immigrant groups want their children to know their (18) culture. Many Hispanics, for instance, want their children to learn (19) English and Spanish and study the Spanish language in school. They are fighting for the (20) to bilingual education in many communities. In many communities they are in the majority.
查看答案